In recent years, electric automobiles each of which drives wheels by an electric motor instead of an internal combustion engine have attracted an increasing interest from the viewpoint of prevention of air pollution and reduction of vehicle noise.
These electric automobiles also include those provided with a transmission so that output rotation from a drive motor is transmitted to drive wheels by selecting a gear position of the transmission and changing the speed of the rotation in accordance with the gear position so selected.
Transmissions of such electric automobiles, like automobiles of the internal combustion type, include manual transmissions whose gear positions are each changed over by a driver's manual operation with disconnection and connection of a clutch, automatic transmissions whose gear positions are each changed over automatically based on a vehicle speed, a motor rotation speed and an accelerator stroke, and semiautomatic transmissions or clutchless transmissions that, although the driver does not perform a clutching operation, a gear ratio is changed over by a manual operation and the transmission is changed over based on this operation under electronic control or the like.
Different from an automobile of the internal combustion type, an electric automobile is not required to perform idling during a stop so that its motor is stopped. An automatic transmission for use in an automobile, which is provided with an internal combustion engine as a drive source, cannot therefore be adopted as is for the electric automobile. Namely, in the case of a hydraulically-driven transmission, for example, an oil pump which produced a hydraulic pressure is normally driven by a drive motor. In an electric automobile, a motor however remains stopped during a stop so that no hydraulic pressure is produced. It is therefore expected that a change-over operation of a transmission may not be performed.
On the other hand, a synchronizer mechanism which is adopted in a manual transmission or a manual transmission equipped with an automatic clutch has low practical utility in an electric automobile due to the high potential problem that a change-over of a gear position may not be feasible.
A transmission different from those employed in automobiles of the internal combustion type is therefore needed.
Further, a large shift shock tends to occur upon completion of a change-over of a gear position of a transmission because of a difference in speed between an output rotation speed of the motor and a wheel speed. It is therefore necessary to make an output rotation speed of the motor appropriate upon shifting.
Various techniques have therefore been proposed, including techniques that feature addition of such rotation speed control to change-over control of a gear position in an automatic transmission or semi-automatic transmission and techniques that permit control of a motor rotation speed even in the case of a manual transmission.
For example, the technique (first technique) of Japanese Patent Publication (Kokoku) No. SHO 52-17300 relates to a clutchless change-over mechanism which upon production of a shift command, changes a transmission once into a neutral position to accelerate or decelerate a motor and, while detecting a rotational speed of an input gear, makes speeds of the motor and the input gear equal to each other, and then brings gears into engagement, whereby a shift shock is reduced.
The technique (second technique) disclosed, for example, in Japanese Patent Application Laid-Open (Kokai) No. SHO 58-176504 is to achieve matching in speed between a motor and an axle by cutting off a supply of electric power to the motor when the motor and the axle are disconnected by an operation of a clutch pedal or a transmission.
However, the first conventional technique develops a delay in time upon performing control in such a way that a rotational speed of an input gear and that of an output gear are detected and the transmission is then controlled to make these speeds become equal to each other. It therefore takes substantial time until their rotational speeds are made equal to each other. Shifting cannot be performed quickly, thereby resulting in a substantial delay of completion of an actual shifting operation relative to the driver's shifting operation. Normally (i.e., at a time other than a shifting time), only output control of a motor (torque control) is conducted in accordance with an accelerator stroke so that control of a torque is only needed as motor control. According to this technique, however, two logics have to be equipped for the control of a motor, one for torque control and the other for rotational speed control.
On the other hand, the second conventional technique requires as a premise that the accelerator stroke is none (in other words, the motor torque is none) when a gear position is changed over. A motor torque is therefore once brought to zero upon changing a gear position and after completion of the change-over of the gear position (i.e., after engagement of gears or connection of a clutch), a motor torque is controlled again so that it corresponds to a degree of depression of an accelerator by a driver (i.e., an accelerator stroke). When a motor torque is once brought to zero as described above, the vehicle speed decreases. Any attempt to promptly control the motor torque according to the accelerator stroke therefore makes the running of the vehicle awkward, thereby making it impossible to perform smooth running at a time of shifting in some instances.
Of course, shifting cannot be performed if the accelerator is kept depressed.
With the foregoing problems in view, the present invention has as an object the provision of a gear shift control method for an electric automobile, which permits a smooth change-over of a gear position even when an accelerator pedal is kept depressed.